1. Field of the Invention
The invention relates to a process for the synthesis of hydrogen cyanide using a starting-gas stream containing methane or a methane-containing natural-gas stream, ammonia and oxygen.
2. Discussion of the Background
The synthesis of hydrogen cyanide (prussic acid; hydrocyanic acid) by the Andrussow method is described in Ullmann's Encyclopedia of Industrial Chemistry, Volume 8, VCH Verlagsgesellschaft, Weinheim, 1987, pp. 161–162. The starting-gas mixture, which comprises methane or a methane-containing natural-gas stream, ammonia and oxygen is passed into a reactor over catalyst gauze and reacted at temperatures of about 1000° C. The necessary oxygen is usually introduced in the form of air. The catalyst gauzes comprise platinum or platinum alloys. The composition of the starting-gas mixture corresponds approximately to the stoichiometry of the overall equation of the exothermic reactionCH4+NH3+3/2O2→HCN+3H2O dHr=−473.9 kJ.
The discharged reaction gas contains the product HCN, unreacted NH3 and CH4, the main by-products CO, H2, H2O and CO2, and a large proportion of N2.
The reaction gas is cooled rapidly to about 150 to 200° C. in a waste-heat boiler and then passed through a scrubbing column, in which the unreacted NH3 is removed with dilute sulfuric acid and some of the water vapor is condensed. Also known is the absorption of NH3 with sodium hydrogen phosphate solution followed by recycling of the ammonia. HCN is absorbed in cold water in a subsequent absorption column and then purified to better than 99.5 wt % in a downstream rectification unit. The HCN-containing water present in the column bottoms is cooled and recycled to the HCN absorption column.
A broad spectrum of possible embodiments of the Andrussow method is described in German Patent 549055. In one example, a catalyst comprising a plurality of fine-mesh gauzes of Pt with 10% rhodium disposed in series is used at temperatures of about 980 to 1050° C. The HCN yield is 66.1% relative to the feed NH3.
A method for maximizing the HCN yield by optimal adjustment of the air/natural gas and air/ammonia ratios is described in U.S. Pat. No. 4,128,622.
In addition to the standard operating procedure with air as the oxygen supply, the use of oxygen-enriched air is described in various documents such as: German Patent 1283209, which corresponds to Netherlands Patent 6604519, Belgian Patent 679440 and U.S. Pat. No. 3,379,500; German Examined Application 1288575, which corresponds to Netherlands Patent 6604697 and Belgian Patent 679529; International Patent WO 97/09273; and U.S. Pat. No. 5,882,618. Table 1 lists some patents with the operating conditions cited therein.
TABLE 1List of various patent claims regarding oxygen enrichmentGermanExaminedInternationalGerman PatentApplicationPatent WOU.S. Pat. No.1283209, 1968,1288575, 1968,97/09273, 1997,5882618, 1999,Società EdisonSocietà EdisonICIAir Liquidecorresponds toNetherlandsPatent 6604519,Belgian Patent679440NetherlandsU.S. Pat. No.Patent 6604697,3379500Belgian Patent(italics)679529special reactorStarting-gas—  200 to 400° C.200 to 400° C.preheating  300 to 380° C.furtherGauze 1100 to 1200° C. 1100 to 1200° C.temperature data1100 ± 50° C.temperaturefor individualstarting-gasstreams(O2 + N2)/CH4 6.5 to 1.55 6.0 to 1.6Ratios reportedmolar ratio 4.55 to 2.80 4.5 to 2.6as relative to the(O2 + N2)/NH3 6.8 to 2.0 6.0 to 2.0mode of 4.8 to 3.65 4.5 to 3.0operation with airCH4/NH3 1.4 to 1.05 1.3 to 1.0 1.0 to 1.5 1.3 to 1.1 1.25 to 1.05O2/(O2 + N2)0.245 to 0.40.245 to 0.35 0.3 to 1.0 0.27 to 0.317 0.25 to 0.30
U.S. Pat. No. 5,882,618 describes the synthesis of hydrocyanic acid by the Andrussow method using oxygen-enriched air. To circumvent the problems that occur under these conditions, such as proximity to the explosion limits of the mixture of NH3, CH4 and oxygen-enriched air, as well as the elevated temperature of the catalyst gauze, which can lead to yield losses and shortened catalyst life, the following measures are proposed:
In a first process step, the system is started up with air as the oxygen source. During this first process step, the catalyst mesh reaches a well-defined temperature.
In a second process step, oxygen is then added and at the same time the contents of ammonia and methane are adjusted such that the mixture is situated above the upper explosion limit and the catalyst temperature corresponds to within 50 K of the reference temperature determined in step 1. The temperature of the catalyst gauze is about 1100° C. to 1200° C. By means of this procedure, safe use of the system is achieved during operation with oxygen-enriched air.
International Patent WO 97/09273 overcomes the disadvantages of high N2 dilution of the reaction gases by the use of preheated, mixtures of methane, ammonia and oxygen-enriched air or pure oxygen capable of detonation.
The enrichment with oxygen of the starting gas for HCN synthesis according to Andrussow, described heretofore, has the following disadvantages:                proximity to the upper explosion limit of the starting gas mixture (danger of explosions, deflagrations and local temperature spikes, resulting in damage to the catalyst gauze);        low yield relative to NH3;        higher catalyst temperature and faster deactivation;        maximum O2 enrichment in the standard Andrussow reaction is up to 40% O2 in air;        high investment and maintenance costs for the use of special reactors (International Patent WO 97/09273).        
The advantages of enriching the starting-gas stream with oxygen are essentially the following:                increased productivity (kg HCN per hour) in existing plants through reduction of the inert gas concentration; and        lower energy consumption for HCN absorption and rectification.        
By adjusting conditions and concentration ratios in the starting gas to correspond to the solution described in the claims, the advantages of enrichment with oxygen can be achieved without having to tolerate the described disadvantages.